Solvent recovery method



Oct. 24, 1.939. w. w. KRAFT r-:T AL

SOLVENT RECOVERYA METHOD Original Filed May 1. 1935 @am Emana ZOFUFXU i N ms :s

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SOLVENT RECVERY MEEHEE Wheaton W. Kraft and Merle A. Zierman, New York, N. Y., assignors to The Baumhaus Company, New Yaris, N. Y., a corporation of Delaware appucauon'May i, i935, seran ne. raars Renewed .gully 6, 1939 Claims.

The present invention relates to slvent recovery methods.

In many industrial processes, substantially complete recovery of solvent for re-use in the 5 process is often attended with some difficulty. Frequently the solvent may be recovered from the extracted material by steam stripping but as is sometimes the case, the solvent is of a character to form a constant boiling mixture with water, l0 and the dimculty is then in preventing loss oi' solvent in the water ultimately removed from the system.

The principal object or the present invention is to provide a method for handling' such mixl'tures without more than a negligible loss of solvent.- To this end, thepresent invention comprises a method suitable for use with certain types of solvents whereby the solvent itself and the water used for steam stripping may be circulated in a closed cycle.

the separation of a solvent .such as cresylicvv acid or phenol from a mixture thereof with oils lobtained in solvent extraction of petroleum.

'Ihe extraction apparatus illustrated diagrammatically at 2 gives as one end product a solution of certain oils in cresylic acid. These oils i 40 are commonly referred to as extract. There is also obtained from the extraction apparatus a product consisting of parafllnic oils containing an appreciable amount of solvent. These oils are commonly referred to as ralnate. However,

i since the separation of the solvent from either the extract or the raffinate depends on the same principles, no distinction is herein made, and the system is described as operating on a, mixture oi' cresylic acid solvent with extracted oils, whether such extracted oils be so-called extract or rahnate.

The mixture of cresylic acid and oils `is fed from the extraction apparatusby a line 3 through a. pump '4 and heater 6 into a column 8 4whereiny u steam is introduced 'at the` bottom through a vrecovery column.

The residue from the column 8, comprising oil in mixture with some solvent, is pumped by a line it through a heater l into a second distilling ,.0 column 2t, preferably maintained under vacuum. Steam is introduced through a steam line 2 i. The oil, practically free from solvent, is Withdrawn from the bottom of the column by a line tl. The vapors comprisingr cresylic acid and steam are EQ condensed in a condenser 2&3, a part of the condensate being returned to the column 263 as rcux through the pipe 2t and the remainder being collected in a tank @EL The two columns il and t@ provide for vtwo-stage separation of the solvent from the oil without the necessity for overheating the materials, although a single column may be used for this purpose if desired.'

Cresylic acid and water form an aaeotropic mixture. The condensate collected at @El therefore 2o is subjected to further treatment, and to this end, the mixture is lpumped from the tank 38 by a line 32 into a mediate point of the solvent recovery column ifi where it contacts with the solvent and Water vapors introduced by the pipe Sii l2. The rectifying action between the vapors lntroduced at l2, the liquid introduced at 32 and reux from the top, results in the accumulation of dry solvent at the bottom of the column lf3 which is pumped out of the recovery column by 85 a line 3@ into a solvent tank 35. 'I'he vapors of solvent and water leaving the top of the recovery column lli constitute an azeotropic mixture of minimum boiling point. This mixture is condensed and cooled in a condenser 36, a portion of 4o the condensate being returned to the column lf3 -as reilux through the line 38 and the remainder being collected in a tank 4D.

The proportions of water and solvent in the constant boiling mixture are not absolutely de- 45 termined since cresylic acid is a Yphenol-cresol mixture and not a definite chemical compound, but with cresylic acid within the ordinary range of composition, the constant boiling mixture contains from 5 to 10% of cresylic acid. This mix- 50 ture has a boiling point lower than that of either cresylic acid or water alone and hence is obtained as the distillate while the dry solvent is obtainedv as residue in the column I4. Upon con- Adensation of the distillate vapors and cooling to 'I'he'materials of the two layers are now used directly in other steps of the process. The

.. solvent layer containing 15 to 20% of water is run to the solvent tank 35 by a pipe. 42 to be mixed with the dry solvent obtained at 34 and the whole mixture is re-'used in the solvent extraction process.l A line 43 connects the solvent tank with the extraction apparatus 2. It will be understood that the greater part of the solvent is recovered as dry solvent at 34 so that the percentage of water in the entire mixture is relatively small.

The upper or water layer in the vessel 40| containing 2 to 3% of cresylic acidv is used for stripping steam in the distilling columns 8 and 20. To this end, the water layer is 'pumped from the vessel 40 through a pipe 44 and heated by a heat exchanger or indirect heater 46 to generate steam. 'I'he pipes I0 and 2| leading to the bottom of the columns 8 and 20, respectively,'are fed with steam from the heater 46. A draw-olf pipe 41 is connected to the pipe 44 for voiding water from the system, as will hereinafter be described.

In the specic example above given, the percentage of cresylic acid in the steam introduced int'othe dlstilling columns 8 and 20 is not large enough to have any material influence on the distillation. If the water layer contains 2.4% of cresylic acid, the molal percentage of cresylic acid will be only about 0.4, since the molecular weights of the two materials arevin the ratio oi about 6 to 1. The partial pressure ofthe cresylic acid on steam at 760 mm. absolute pressure is therefore only about 3 mm. of mercury. Under these conditions, the solvent may be almost completely stripped from the oil with steam generated from the water layer.

It will be seen that the materials obtained in the vessel 40 are continuously circulated in a a closed cycle and that therefore if no water enters the system from any external source, the process could be carried on indeiinitely as above described with only such make-up of solvent as is necessary to compensate for unavoidable losses occurring in any distilling method. However,-

sqme water unavoidably enters the system, either with the charged crude oil or in some other manner. Although such water as may be introduced in this way is small in quantity, it cannot be permitted to remain indenitely since it'would lultimately build up to a considerable amount.

'I'he purpose of the draw-off pipe 41 is to void the system of such water which 'may be: done either continuously or intermittently. 'I'he water removed at 41 contains 2 to 3% of solvent and this represents a loss of solvent, but since the water is extremely small in amount, the solvent loss is negligible.

Although the invention has been described as applied to a process for solvent extraction of petroleum oils, the invention is not limited to such process. The apparatus 2 may be any con-- tact apparatus resulting in mixture of a solvent and an extracted material. d

For application of the principles of solvent recovery according to the present invention,'the

solvent must be one which forms a constant boil-` ing' mixture with water and which is miscible, at least partially, with water. If the solvent does not form a constant boiling mixture with water, the recovery can be effected by simple distillation. Many of the solvents used in industrial processes, however. are not amenable to recovery by simple methods, and the present inventlon may then be applicable.

It will be seen that the invention is of the greatest value when the ultimate condensate comprisesy either the solvent saturated with 'a small quantity of water or water saturated with a-small quantity of solvent; in the former case.

the mixture may be used in conjunction withvunsaturated solvent for further extraction, and in the latter case, the mixture may be used as process steam. In the specific example abovegiven, both oi these conditions arise, since the condensate divides into two layers oi widely different lproportions which are available for separate rea stripping medium to' strip solvent from thel original solution, iractionating the stripped vapors of solvent and Water substantially between dry solvent as residue and said azeotropic mixture as distillate, and condensing and cooling the distillate vapors to form the above mentioned solvent layer and water layer and thereby to complete the closed cycle of the water layer.

2. In a method of recovering a solvent from a solution of an organic material therein, said solution containing a small quantity of water, the solvent being of a character to form with water an azeotropic mixture condensable in a water layer containing a lsmall proportion of solvent and a solvent layer containing a small proportion of water, the steps which consist in circulating the material of the water layer in a closed cycle by vaporizing said material and utilizing it as a stripping medium to strip solvent 'from the original solution, fractionating the stripped vapors of solvent and water substantially between dry solvent as residue and said azeotropic mixo ture as distillate, condensing and cooling the distillate vapors to form the above mentioned solvent layer and water layer and thereby to complete the closed cycle of the water layer, and voiding from said closed cycle a small portion o! the material of the water layer to compensate for the water contained in the original mixture.

3. In a method of recovering' a solvent from a mixture thereof with extracted petroleum oils, the solvent being of a character to form with water an azeotropic mixture condensable in a water layer containing a small proportion of solvent and a solvent layer containing a small proportion oi water, the steps which consist in circulating the material of, the water layer in a closed cycle by vaporizing said material and utilizing it as a stripping medium to strip solvent from the oils, fractionating the stripped vapors of solvent and water substantially between dry solvent las residue and said azeotropic mixture as distillate, and condensing and cooling the distillatevapors to form the above mentioned solventv a mixture thereof with extracted petroleum oils, the' solventbeing of a character to form with water an azeotropic mixture condensable in a water layer containing Va small proportion of solvent and a-solvent layer containing a small proportion of water, the steps which consist in, circulating the material of the water layer in av closed cycle by vaporizing said material and utilizing it as a stripping medium to strip solvent from the oils, fractionati'ng the stripped vapors of solvent and water substantially between dry solvent as residue and said azeotropic mixture vas distillate, condensing and cooling the distil late vapors to form the above mentioned solvent layer and water layer and thereby to complete` the closed cycle of the water layer, and voiding from said closed cycle a small'portion of the material of the water layer to compensate for the water contained in the original mixture.

5. In a method of solvent extraction and recovery withV a solvent which is of a character to form with water an azeotropic mixture condensable in a Water layer containing a small proportion of solvent and a solvent layer containing a small proportion of water, the steps which consist in circulating the material of the solvent layer in a closed cycle through an extraction zone for contact with petroleum oils, a stripping zone for separation of the solvent from the f oils, a fractionating zone and condensing zone,

.circulating the material of `the water layer in a l second closed cycle by vaporizing it and passing it as a stripping medium .to the stripping zone and thence to thefractionating and condensing Y zones, the strippedvvapors of solvent and water being fractionated substantially between dry solvent as residue and said azeotropic mixture as distillate, and the distillate vapors being condensed and cooled in the condensing zone to `form the above mentioned solvent layer and lWater layer. i Y

6. In a ymethod of recovering cresylic acid solvent from a mixture thereof with extracted petroleum oils, the solvent being of acharacter to form with water an azeotropic mixture condensable in a water layer containing a small quantity of cresylic acid and a solvent layer containing a small quantity of water, the steps which consist in circulating the material of the water layer in a. closed cycle by vaporizing said mateto form the above mentioned solvent layer and water layer and thereby to .complete the closed cycle of the waterlayer. c

'7. In a method of solvent extraction and recovery with a solvent which is of a character to form with water an azeotropic mixture condensable in a waterlayer containing a small proportion of solvent and a solvent layer containing a small proportion of water, the steps which consist in circulating the 'material of the water layer in a closed cycle by vaporizing said material and utilizing it yas a stripping medium to strip solvent from the original mixture, fractionating the stripped vapors of solvent and water substantially between drysolvent as residue and said azeotropic mixture lvas distillate, condensing and cooling the distillate/vapors to. form the above mentioned` solvent layer' and water layer and thereby to complete the closed cycle of the water layer, and voiding from said closed cycle a small portion of ,the material of the water layer to compensate for the waterv contained in the original mixture.

8. In a method of solvent extraction of a solution of an organic material therein, the solvent material being of a. character to form with water an azeotropic mixture condensable in a Water layer containing a preponderant amount of water, the steps which consist in circulating4 stripping medium to strip solvent from the original solution, fractionating the stripped vapors of solvent and water. substantially between dry solvent as residue and distillate vapors as overhead, approaching said azeotrope in composition, and condensing andv cooling the distillate vapors to form the waterY layer, and thereby to complete the closed cycle oi the water layer.

9. In a method of recovering a solvent from a solution ofwan organic material, as claimed in claim 8,'the step of removing a substantial pore tion of the solvent from the overhead distillate prior to the vaporizing o f the distillate .vapor condensate for stripping steam.

1'0. In a method of recovering a solvent from a solution of an organic material, as claimed in claim 8, in which'the azeotropic mixture of solvent and water are condensable in two layers, the second of which is preponderantly solvent,

Iand said vtwo layers `are decanted to remove the WHEATON W. KRAFT. MERLE vA. ZIIMMERMAN. 

